This invention relates to devices, particularly optical devices, for controlling propagation of energy, particularly optical beams, using electric field control. In particular, the invention relates to devices with poled structures, including periodically poled structures, and electrodes which permit controlled propagation of optical energy in the presence of controlled electric fields applied between electrodes. The invention relates to a fundamentally new class of flat panel optical displays.
The current technology for an EO switchable grating is shown in FIG. 1 (Prior Art). In this structure, periodically patterned electrodes serve as the elements that define the grating. The underlying material does not have a patterned poled structure, as hereinafter explained. An input beam 12 is coupled into a electro-optically active material 2 which contains an electrically controllable permanent grating 6. When the voltage source 10 to the grating electrodes is off, the input beam continues to propagate through the material to form the output beam 16. When the grating-controlling voltage source is switched on, an index modulation grating is produced in the material, and a portion of the input beam is coupled into a reflected output beam 14. The material has an electro-optically active poled region 4 with a single domain, with the same polarity throughout the poled structure. A first electrode 6 is interdigitated with a second electrode 7 on a common surface 18 of the substrate. When a voltage is applied between the electrodes, the vertical component of electric field along the path of the beam 12 alternately has opposite sign, creating alternate positive and negative index changes to form a grating. The strength of the grating is controlled by the voltage source connected between the two electrodes by two conductors 8.
A second general problem with the existing art of EO and piezoelectric devices using uniform substrates and patterned electrodes is that the pattern of the excited electric field decays rapidly with distance away from the electrodes. The pattern is essentially washed out at a distance from the electrodes equal to the pattern feature size. This problem is aggravated in the case of a grating because of the very small feature size. Prior art gratings formed by interdigitated electrodes produce a modulated effect only in a shallow surface layer. EO structures interact weakly with waveguides whose dimension is larger than the feature size. While longer grating periods may be used in higher order interaction devices, the lack of sharp definition described above again seriously limits efficiency. The minimum grating period for efficient interaction with current technology is about 10 microns. What is needed is a way to maintain the efficiency of EO devices based on small structures, despite a high aspect ratio (i.e. the ratio of the width of the optical beam to the feature size). Switchable patterned structures are needed which persist throughout the width of waveguides and even large unguided beams.
There are several related technologies in the prior art that use light sources coupled with waveguide structures for display applications.
J. Viitanem and J. Lekkala ("Fiber optic liquid crystal displays," SPIE Vol. 1976, High-Definition Video, pg. 293-302 (1993), and references therein) review the characteristics of flat panel displays that use the waveguide principle coupled with liquid crystal switching. A number of designs are discussed. All have the following common design principles. A modulated light source is mechanically scanned across a series of electro-optically active waveguides that form the row elements of the display. A series of parallel electrodes form the column locations for the display. Light is coupled out of the waveguides and scattered toward the viewer at a column spatial location using the electro-optic effect. Thus a two-dimensional array of pixels is formed.
In this prior art, light is confined in a waveguide which is composed of a core optical material that has an index of refraction that is larger than the surrounding cladding material. The light, normally confined primarily to the core, is forced to "leak" out of the core of the waveguide at a desired spatial location. The waveguiding effect is destroyed by electro-optically reducing the index difference between the core and the cladding along a certain distance. The electro-optically active material may, in principle, either reside in the core (to reduce the index) or in the cladding (to increase the index. In Viitanem et al., the cladding is active. The technique of destroying the waveguiding effect is called a "waveguide tap" in some of the prior art literature.
The "leaked" light propagates by free space diffraction to a scattering center where it is directed toward the viewer to form a pixel of the display. The light that "leaks" out of the destroyed waveguide is no longer spatially confined but expands in area according to standard diffraction theory as it propagates away from the destroyed waveguide segment. This two-dimensional expansion of the light causes three problems.
First, since the diffraction angle of the light previously confined to the waveguide is relatively small, a long interaction lengths results. (A significant fraction of the optical energy must leave the core region before it can be scattered toward the viewer.) This typically will limit the spacing of the scattering centers to be larger than 1 mm. This effect causes a low resolution display with a low pixel packing density.
Second, the two dimensional expansion of the beam makes it virtually impossible to collect a large fraction of the light on a scattering center and direct it toward the viewer. This causes the display to have a low electrical power efficiency.
Third, the two-dimensional expansion of the beam causes the scattering centers to be large, and hence the pixel size is large. This also degrades the display resolution.
A consequence of the large pixel spacing is that long waveguide lengths must be used to cover enough pixels for a display. The display must then operate in a region where the effects of waveguide loss are large, again reducing efficiency. Thus this prior art design suffers from low pixel packing density, a large pixel size, and a low electrical power efficiency.
What is needed to resolve these problems is the development of a short, efficient, low-loss electro-optic waveguide switch that routes the entire light beam out of the row waveguide and into a narrow solid-angle so that the switched light can be efficiently directed either towards a pixel scattering center or into another waveguide that leads to a scattering center. This will concentrate the light on the scattering center, maximizing both the efficiency of the display and the pixel packing density.
Another embodiment using the "waveguide tap" method is described in U.S. Pat. No. 5,106,181, April/1992, and U.S. Pat. No. 5,009,483, April/1991, Rockwell, III, "Optical Waveguide Display System". Rockwell III discloses a display that uses waveguides to guide light in simultaneous rows. Light is coupled out of a waveguide into the cladding using the electro-optic effect in the cladding. Although the structure is different from that of Viitanem et al. above, this display suffers the same problems of low pixel density and inefficiency discussed above.
U.S. Pat. No. 5,045,847, September/1991, Tarui et. al., "Flat Display Panel", discloses another version of the "waveguide tap" method. A planar waveguide structure is used with a layered core material consisting of interspersed layers of a-SiN and a-Si. Light from a laser diode source is confined within the planar waveguide until a voltage is placed across the core of the waveguide. The voltage causes the index of the core to be reduced thus allowing light to escape the waveguide structure. This display suffers from all of the difficulties discussed above. This design has an additional efficiency penalty since all pixels are simultaneously illuminated, but only one pixel at a time is activated. Thus a very small fraction of the light is coupled out toward the viewer at any one time.
U.S. Pat. No. 5,083,120, January/1992, Nelson, "Flat Panel Display Utilizing Leaky Lightguides" discloses a leaky lightguide used as a row-backlight for a display. Light from a source such as a laser diode is coupled into the lightguide to provide uniform illumination of a row in a display. This backlight is combined with an array of ferroelectric liquid crystal shutters to make the display pixels. In this case the waveguides are only used to replace the fluorescent light normally used in LCD displays, and there are no active "waveguide taps" or switches.
U.S. Pat. No. 4,640,592, February/1987, Nishimura et al., "Optical Display Utilizing Thermally Formed Bubble in a Liquid Core Waveguide", discloses a display that uses multiple liquid filled fibers or waveguides as rows. These waveguides are placed over a series of heater electrodes. Bubbles are formed in the waveguide core when the heater voltage is turned on, thus scattering light out toward the viewer. The display image is formed when modulated laser light is mechanically scanned sequentially into each row waveguide. Although this approach solves some of the problems inherent in the "waveguide tap" approach by putting small scattering centers in core region, the time scale of thermal processes will preclude this display from having a fast enough frame rate to display full motion video. There are no waveguide switches in this design.
All of the prior art uses parallel input waveguides excited directly by a light source because the switching mechanisms known to the art do not permit the efficient switching light out of a supply waveguide into the desired row waveguide. This accounts for the unwieldy mechanical beam scanning apparatus of Viitanem et al. and Nishimura et al., the simultaneous waveguide bundle illumination of Rockwell III, the planar waveguide excitation of Tarui et al., and the simultaneous diode array illumination of Nelson and Nishimura et al. Yet the simplest architecture for a display, where light is coupled into a single waveguide and routed to the pixels, requires at least two consecutive switches to illuminate a full two dimensional array of pixels: one to select the row waveguide and the other to select the pixel. The prior art can (with difficulty) make the pixel switches. However, the row switch, which must connect waveguide to waveguide, is not possible with the waveguide taps described above. Waveguide to waveguide coupling would just be too inefficient because of the divergence of the light by two dimensional diffraction following the destruction of the waveguide in the switched region. What is needed is a switch which concentrates the switched light efficiently into another waveguide.
The integrated optical channel waveguide communication bus of Becker and Chang, can not be used as a display since it does not include "waveguide taps" or any other method to allow the light propagating in the waveguides to be directed toward a viewer in a pixel format. Furthermore, the switches are fabricated using an interdigitated electrodes structure previously described above in reference to FIG. 1, and they do not contain any pattern poled structures. The switch design has a high switch insertion loss, and hence can not be used in any applications which require a large number of switches on the same waveguide such as a display or a multi-switch data bus for communication applications.
The problems with the prior art are summarized as follows: 1) compact high resolution displays can not be fabricated due to the large pixel spacing, 2) the inefficient "waveguide taps" limit the brightness of the display, 3) the power efficiency is low because of the optical losses, and 4) power cannot be efficiently switched into another waveguide.